In industrial communication networks of distributed control systems, reliability or availability is a key issue, because a failing communication network generally entails an interruption of the controlling process, which might lead to a shut down of the controlled industrial process. The fast identification of a failed or otherwise inoperable component of the industrial communication network is thus important. This is particularly valid for the field of Substation Automation (SA), as the reliability of a SA system has an immediate impact on the reliability of an entire power network.
Substations for power distribution in high and medium voltage power networks include primary or field devices such as electrical cables, lines, busbars, switches, breakers, power transformers and instrument transformers. These primary devices are operated in an automated way via a Substation Automation (SA) system responsible for controlling, protecting and monitoring of substations. The SA system comprises programmable secondary devices, so-called Intelligent Electronic Devices (IED), interconnected in a SA communication network, and interacting with the primary devices via a process interface. The IEDs are generally assigned to one of three hierarchical levels, namely the station level with the operators place including a Human-Machine Interface (HMI) as well as the gateway to the Network Control Centre (NCC), the bay level with its Bay Units (BU) for protection and control, and the process level. Process level units comprise e.g. electronic sensors for voltage, current and gas density measurements as well as for switch and transformer tap changer positions, or breaker-IEDs controlling an actuator or drive of a circuit breaker or disconnector. Intelligent actuators or breaker IEDs may be integrated in the respective intelligent primary equipment and connected to a Bay Unit via a serial link or optical process bus. The BUs are connected to each other and to the IEDs on the station level via an inter-bay or station bus.
A reliable SA system is typically required to continue to be operable even if a component fails, i.e. a single point of failure that causes the substation to become inoperable is unacceptable. A failure of a component of the SA communication network, due to e.g. a loss of a communication port caused by a failing diode for a fibre optic link, can result in the loss of access to the complete substation, one single bay or only one IED. Albeit the failure rate of the electronic ingredients of an individual IED is already very low, redundancy is one way to increase reliability of the SA system. Redundancy can be achieved by doubling components, or by benefiting from an inherent redundancy of a ring-type communication link. Even in such redundant SA communication networks, it is important to detect errors or failed components, in order to repair or replace the component and re-establish the vital redundancy. Hence, of all the components of the SA communication network, including IEDs, Ethernet switches, Ethernet lines, fibre-optical lines, hubs and opto-electrical star couplers, it is important to know the availability, in other words, a failure of a component has to be localized and diagnosed as quickly as possible in order for a repair or replacement to be arranged for.
In SA communication networks based on the novel Standard IEC 61850, the use of fibre optics and switches overcomes some limitations of the Ethernet like collisions and allowed length extensions. However, opto-electrical star-type couplers working exclusively with electro-optical conversion on a physical level, Ethernet hubs working exclusively with electrical connections, as well as all interconnecting physical lines, are passive components deprived of any intelligence or programmability.
In industrial communication networks with intelligent or active devices such as routers or Ethernet switches, these devices can themselves identify as being out of order or otherwise in need of maintenance. The result of such as self-diagnosis is then transmitted by means of dedicated protocols such as the Simple Network Management Protocol (SNMP) to a monitoring device for evaluation. SNMP is the Internet standard protocol developed to manage nodes (servers, workstations, routers, and switches etc.) on an IP network. The protocol can support monitoring of intelligent network-attached devices for any conditions that warrant administrative attention. However, the particular data transmitted is often device-specific and can only be interpreted correctly by communication experts. In addition, and unless supplemental intelligence is provided for monitoring passive or non-intelligent components, a failure of the latter cannot be identified this way.